Is this guy interested in me? I can't tell!

I have a mad crush on one of my classmates at law school, but I can't tell if he's interested at all. And because I am crushing on him, I am biased and think that everything he does means something. I need an objective view!

Encounter 1: I was alone at a party, and he comes up to introduce himself. We chatted for a bit, but because I was feeling anti-social that night, the convo quickly ended.
Encounter 2: One week later, I go to a social event, and I see him again, but can barely remember him. He comes up to me and asks how my first week of classes went. We again chat for a bit.
Encounter 3: Saw him at a Halloween party. He was dressed up as a condom dispenser (!!), and was selling raffle tickets. I said I'd like to buy 5, and gave him some money.

Him: hey you gave me $6 (it's $1 each)
Me: oh, i'm sorry
Him: well, is that $1 for tip? or maybe you'd like to buy a condom?
Me: *unable to think of a witty comeback and so laughs like an idiot*

I can't tell. Was he being flirty? But if he were interested, I think he'd ask for my number by now, no? What do you think? Is he interested in me?

P.S. After that brief exchange at the halloween party, he didn't approach me again. But then again, I was always surrounded by a bunch of friends...